Recently, various antibody drugs for treating cancers by targeting antigen proteins on cancer cells have become up in the world. The antibody drugs show certain beneficial effects as cancer-specific therapeutic agents and have received attention. However, most of the target antigen proteins are expressed also on normal cells, and administration of such an antibody impairs not only cancer cells but also normal cells expressing the antigen, resulting in a problem of side effects therefrom. Accordingly, if a cancer antigen being specifically expressed on cancer cell surface is identified and an antibody targeting the antigen can be used as a pharmaceutical agent, treatment with an antibody drug with less side effects can be expected.
It is known to those skilled in the art as general technical knowledge that gallbladder cancer, among various cancers, is very difficult to be detected at an early stage because of its lack of symptoms and early symptoms; advanced gallbladder cancer such as lymph node metastasis, liver metastasis, lung metastasis, bone metastasis, or peritoneal metastasis is very difficult to treat, leading to a five-year survival rate of almost 0% for patients with gallbladder cancer not amenable to surgery; and gallbladder cancer is very difficult to treat and effective therapies for the cancer have not been developed.
Cytoplasmic- and proliferation-associated protein 1 (CAPRIN-1) has been known as an intracellular protein that is expressed in activation of normal cells in the resting phase or in occurrence of cell division and is involved in control of transport and translation of mRNA through formation of intracellular stress granules with RNA in cells. It was found that CAPRIN-1 is specifically expressed on the surface of cancer cells such as breast cancer cells, and CAPRIN-1 has been studied as a target of antibody drugs for cancer therapy (Patent Literature 1). However, in Patent Literature 1, expression of CAPRIN-1 protein on gallbladder cancer cells is not recognized, and it is not described or suggested that CAPRIN-1 protein can be an antigen protein of gallbladder cancer.